A cylinder head integrally formed with an exhaust system manifold has been conventionally developed, wherein multiple exhaust ports connected to combustion chambers of an engine merge inside the cylinder head. Such a cylinder head is advantageous in that a shorter distance between an exhaust purification catalyst provided in the exhaust system and the engine improves the performance of the exhaust purification, and that a shorter exhaust system per se reduces the pressure loss of the exhaust and enhances the size reduction of the engine. Such a cylinder head, on the other hand, has a disadvantage in that the temperature may be increased due to exhaust heat, as compared to a cylinder head provided separately with a manifold. To address this issue, techniques have been proposed to improve the cooling performance by permitting engine cooling water (coolant) to flow around an exhaust port and in the vicinity of an outlet of a manifold (refer to Japanese Laid-open Patent Publication No. 2008-309158).
In the meantime, the temperature tends to rise at an outlet of a manifold contained in a cylinder head particularly because exhausts from exhaust ports tend to converge in the vicinity of the outlet of the manifold. Because downstream side exhaust pipes are fastened and secured to outlets of a manifold, there is a need for a structure that can efficiently cool the vicinity of the outlet, for the purpose of suppressing a reduction in the clamping force of fastening members, thereby maintaining a stable clamping force.